1. Field of the Invention
The present invention relates to synthetic fibers especially useful in the manufacture of nonwoven fabrics. In particular, the present invention relates to fibers intended for such use, including processes of their production, and compositions for producing the fibers, as well as nonwoven fabrics and articles containing these fibers. More specifically, the fibers of the present invention are capable of providing soft feeling nonwoven materials that have high tensile strength. Further, the nonwoven materials are thermally bondable at lower temperatures while having superior strength properties, including cross-directional strength. The fibers of the present invention can be incorporated into lower basis weight nonwoven materials which have strength properties that are equal to or greater than nonwoven materials of higher basis weight. Still further, the fibers of the present invention are capable of being run on high speed machines, such as high speed carding and bonding machines.
2. Background Information
The requirements of nonwoven fabrics used in applications concerned with hygiene, medical fabrics, wipes and the like continue to grow. Moreover, utility and economy, and aesthetic qualities often must be met simultaneously. The market continues to expand for polyolefin fibers and items made therefrom having enhanced properties and improved softness.
The production of polymer fibers for nonwoven materials usually involves the use of a mix of at least one polymer with nominal amounts of additives, such as stabilizers, pigments, antacids and the like. The mix is melt extruded and processed into fibers and fibrous products using conventional commercial processes. Nonwoven fabrics are typically made by making a web, and then thermally bonding the fibers together. For example, staple fibers are converted into nonwoven fabrics using, for example, a carding machine, and the carded fabric is thermally bonded. The thermal bonding can be achieved using various heating techniques, including heating with heated rollers, hot air and heating through the use of ultrasonic welding.
Fibers can also be produced and consolidated into nonwovens in various other manners. For example, the fibers and nonwovens can be made by spunbonded processes. Also, consolidation processes can include needlepunching, through-air thermal bonding, ultrasonic welding and hydroentangling.
Conventional thermally bonded nonwoven fabrics exhibit good loft and softness properties, but less than optimal cross-directional strength, and less than optimal cross-directional strength in combination with high elongation. The strength of the thermally bonded nonwoven fabrics depends upon the orientation of the fibers and the inherent strength of the bond points.
Over the years, improvements have been made in fibers which provide stronger bond strengths. However, further improvements are needed to provide even higher fabric strengths at lower bonding temperatures and lower fabric basis weight to permit use of these fabrics in today""s high speed converting processes for hygiene products, such as diapers and other types of incontinence products. In particular, there is a need for thermally bondable fibers, and the resulting nonwoven fabrics that possess high cross-directional strength, high elongation and excellent softness, with the high cross-directional strength (and softness) being obtainable at low bonding temperatures.
Further, there is a need to produce thermally bondable fibers that can achieve superior cross-directional strength, elongation and toughness properties in combination with fabric uniformity, loftiness and softness. In particular, there is a need to obtain fibers that can produce nonwoven materials, especially, carded, calendered fabrics with cross-directional properties on the order of at least about 200 to 400 g/in., more preferably 300 to 400 g/in, preferably greater than about 400 g/in, and more preferably as high as about 650 g/in or more, at speeds as high as about 500 ft/min, preferably as high as about 700 to 800 ft/min, and even more preferably as high as about 980 ft/min (300 m/min). Further, the fabrics can have an elongation of about 50-200%, and a toughness of about 200 to 700 g/in, preferably about 480-700 g/in for nonwoven fabrics having a basis weight of from about 10 g/yd2 to 20 g/yd2. Thus, it is preferred to have these strength properties at a basis weight of about 20 g/yd2, more preferably less than about 20 g/yd2, even more preferably less than about 17 to 18 g/yd2, even more preferably less than about 15 g/yd2, and even more preferably less than about 14 g/yd2 and most preferably as low as 10 g/yd2, or lower. Commercial fabrics produced today, depending upon use, have a basis weight of, for example, about 11-25 g/yd2, preferably 15-24 g/yd2.
Softness of the nonwoven material is particularly important to the ultimate consumer. Thus, products containing softer nonwovens would be more appealing, and thereby produce greater sales of the products, such as diapers including softer layers.
Various techniques are known for producing fibers that are able to be formed into nonwoven materials having superior properties, including high cross-directional strength and softness. For example, U.S. Pat. Nos. 5,281,378, 5,318,735 and 5,431,994 to Kozulla are directed to processes for preparing polypropylene containing fibers by extruding polypropylene containing material having a molecular weight distribution of at least about 5.5 to form a hot extrudate having a surface, with quenching of the hot extrudate in an oxygen-containing atmosphere being controlled so as to effect oxidative chain scission degradation of the surface. In one aspect of the process disclosed in the Kozulla patents, the quenching of the hot extrudate in an oxygen-containing atmosphere can be controlled so as to maintain the temperature of the hot extrudate above about 250xc2x0 C. for a period of time to obtain oxidative chain scission degradation of the surface.
As disclosed in these patents, by quenching to obtain oxidative chain scission degradation of the surface, such as by delaying cooling or blocking the flow of quench gas, the resulting fiber essentially contains a plurality of zones, defined by different characteristics including differences in melt flow rate, molecular weight, melting point, birefringence, orientation and crystallinity. In particular, as disclosed in these patents, a fiber produced therein includes an inner zone identified by a substantial lack of oxidative polymeric degradation, an outer zone of a high concentration of oxidative chain scission degraded polymeric material, and an intermediate zone identified by an inside-to-outside increase in the amount of oxidative chain scission polymeric degradation. In other words, the quenching of the hot extrudate in an oxygen containing atmosphere can be controlled so as to obtain a fiber having a decreasing weight average molecular weight towards the surface of the fiber, and an increasing melt flow rate towards the surface of the fiber. For example, a preferred fiber comprises an inner zone having a weight average molecular weight of about 100,000 to 450,000 grams/mole, an outer zone, including the surface of the fiber, having a weight average molecular weight of less than about 10,000 grams/mole, and an intermediate zone positioned between the inner zone and the outer zone having a weight average molecular weight and melt flow rate intermediate the inner zone and the outer zone. Moreover, the inner, core zone has a melting point and orientation that is higher than the outer surface zone.
Further, U.S. patent application Ser. Nos. 08/080,849, 08/378,267, 08/378,271 and 08/378,667 (and its continuation application Ser. No. 08/598,168, which issued as U.S. Pat. No. 5,705,119) to Takeuchi et al., and European Patent Application No. 0 630 996 to Takeuchi et al., which are incorporated by reference herein in their entirety, are directed to obtain fibers having a skin-core morphology, including obtaining fibers having a skin-core morphology in a short spin process. In these applications, a sufficient environment is provided to the polymeric material in the vicinity of its extrusion from a spinnerette to enable the obtaining of a skin-core structure. For example, because this environment is not achievable in a short spin process solely by using a controlled quench, such as a delayed quench utilizable in the long spin process, the environment for obtaining a skin-core fiber is obtained by using apparatus and procedures which promote at least partial surface degradation of the molten filaments when extruded through the spinnerette. In particular, various elements can be associated with the spinnerette, such as to heat the spinnerette or a plate associated with the spinnerette, so as to provide a sufficient temperature environment, at least at the surface of the extruded polymeric material, to achieve a skin-core fiber structure.
Still further, Kozulla, U.S. patent application Ser. No. 08/358,884, filed Dec. 19, 1994, its continuation application Ser. No.08/998,592 and European Patent Application No. 0 719 879, which are incorporated by reference in their entirety, are directed to the production of skin-core fibers that can be produced under various conditions while ensuring the production of thermally bondable fibers that can provide nonwoven fabrics having superior cross-directional strength, elongation and toughness.
Still further, it is known that blends of materials can be extruded to obtain fibers. For example, U.S. Pat. No. 3,433,573 to Holladay et al. is directed to compositions comprising blends of 5 to 95% by weight of a propylene polymer containing a major amount of propylene, and 95 to 5% by weight of a copolymer of ethylene with a polar monomer, such as vinyl acetate, methyl methacrylate, vinylene carbonate, alkyl acrylates, vinyl halides and vinylidene halides. Compositions within the broad scope of Holladay et al. include blends containing 5 to 95% polypropylene and correspondingly, from about 5 to 95% ethylene/vinyl acetate copolymer, expressed as weight percent of the ultimate blend. The compositions of Holladay et al. may be formed into fibers, films and molded articles of improved dyeability and low temperature characteristics.
Moreover, U.S. Pat. No. 4,803,117 and European Patent Application No. 0 239 080 to Daponte are directed to melt-blowing of certain copolymers of ethylene into elastomeric fibers or microfibers. The useful copolymers are disclosed to be those of ethylene with at least one vinyl monomer selected from the group including vinyl ester monomers, unsaturated aliphatic monocarboxylic acids and alkyl esters of these monocarboxylic acids, where the amount of vinyl monomer is sufficient to impart elasticity to the melt-blown fibers. Exemplary copolymers disclosed by Daponte are those of ethylene with vinyl acetate (EVA) having a melt index in the range from 32 to 500 grams per ten minutes, when measured in accordance with ASTM D-1238-86 at condition E, and including from about 10% by weight to about 50% by weight of vinyl acetate monomer, more specifically from about 18% to about 36% by weight of vinyl acetate monomer, and most specifically from about 26% to about 30% by weight of vinyl acetate monomer, with an even more specific value being about 28% by weight.
The copolymer of Daponte can be mixed with a modifying polymer, which may be an olefin selected from the group including at least one polymer selected from the group including polyethylene, polypropylene, polybutene, ethylene copolymers (generally other than those with vinyl acetate), propylene copolymers, butene copolymers or blends of two or more of these materials. The extrudable blend of Daponte usually includes from at least 10% by weight of the ethylene/vinyl copolymer and from greater than 0% by weight to about 90% by weight of the modifying polymer.
WO 94/17226 to Gessner et al. is directed to a process for producing fibers and nonwoven fabrics from immiscible polymer blends wherein the polymer blend can include polyolefins, such as polyethylene and polypropylene. Additionally, the blend may include up to about 20% by weight of one or more additional dispersed or continuous phases comprising compatible or immiscible polymers, for example, up to about 20% by weight of an adhesive promoting additive, which amongst other materials can be poly(ethylene vinyl acetate) polymers.
Still further, it is known that composite fibers, e.g., having a sheath-core or side-by-side structure, can be produced with different polymers in the different components making up the composite fibers. For example, U.S. Pat. Nos. 4,173,504, 4,234,655, 4,323,626, 4,500,384, 4,738,895, 4,818,587 and 4,840,846 disclose heat-adhesive composite fibers such as sheath-core and side-by-side structured fibers which, amongst other features, include a core that can be composed of polypropylene and a sheath that can be composed of ethylene vinyl acetate copolymer.
Further, U.S. Pat. No. 5,456,982 discloses a bicomponent fiber wherein the sheath may additionally comprise a hydrophilic polymer or copolymer, such as (ethyl vinyl acetate) copolymer.
It is an object of the present invention to provide thermal bonding fibers for making fabrics with high cross-directional strength, elongation and toughness.
It is another object of the invention to provide fibers for making nonwoven materials that are softer than those made with polypropylene fibers.
It is another object of the invention to provide polypropylene fibers which thermally bond well at lower temperatures.
It is yet still a further object of this invention to provide polypropylene fibers with a relatively flat bonding curve.
It is an object of the present invention to obtain thermal bonding of fibers at lower bonding temperatures while maintaining high cross-directional strength, elongation and toughness of the resulting nonwoven material.
It is a further object of the present invention to provide a greater bonding window by obtaining a flatter curve of cross-directional strength vs. bonding temperature to permit thermal bonding of fibers at lower bonding temperatures while maintaining high cross-directional strength of the resulting nonwoven material, whereby lower bonding temperatures can be utilized to enable the obtaining of softer nonwoven materials.
It is still a further object of the present invention to provide lower basis weight nonwoven materials that have strength properties, such as cross-directional strength, elongation and toughness that are equal to or greater than these strength properties obtained with other polypropylene fibers at higher basis weights.
It is still a further object of the present invention to provide fibers and nonwovens that can be handled on high speed machines, including high speed carding and bonding machines, that run at speeds as great as about 980 ft/min (300 m/min).
It is still a further object of the present invention to provide biconstituent or multiconstituent fibers having a skin-core structure produced from blends of polypropylene and polymeric bond curve enhancing agent.
In one aspect of the present invention, it is an object to provide a process for preparing a fiber having a skin-core structure, comprising extruding a polymer blend comprising polypropylene and polymeric bond curve enhancing agent as a hot extrudate; and providing conditions so that the hot extrudate forms a fiber having a skin-core structure. The hot extrudate can be extruded in an oxidative atmosphere under conditions to form a skin-core structure.
The process for preparing a fiber having a skin-core structure can also comprise extruding a polymer blend comprising polypropylene and polymeric bond curve enhancing agent as a hot extrudate; and controlling conditions so that the hot extrudate forms a fiber having a skin-core structure.
In one aspect of the present invention, the polymeric bond curve enhancing agent can provide flattening of a bond curve of cross-directional strength vs. temperature as compared to a nonwoven material produced under same conditions from fibers produced under same conditions except for absence of the polymeric bond curve enhancing agent.
In another aspect of the present invention, the polymeric bond curve enhancing agent can provide raising of at least some points of cross-directional strength of a bond curve of cross-directional strength vs. temperature as compared to a nonwoven material produced under same conditions from fibers produced under same conditions except for absence of the polymeric bond curve enhancing agent, with the raising of at least some points of cross-directional strength preferably including raising of peak cross-directional strength or raising at least some points at temperatures lower than peak cross-directional strength.
In still another aspect of the present invention, the polymeric bond curve enhancing agent can provide raising of at least some points of cross-directional strength and shifting to lower temperatures of a bond curve of cross-directional strength vs. temperature as compared to a nonwoven material produced under same conditions from fibers produced under same conditions except for absence of the polymeric bond curve enhancing agent, with the raising of at least some points of cross-directional strength preferably including raising of peak cross-directional strength or raising at least some points at temperatures lower than peak cross-directional strength.
In still another aspect of the present invention, the polymeric bond curve enhancing agent provides flattening, raising of at least some points of cross-directional strength, and shifting to lower temperatures of a bond curve of cross-directional strength vs. temperature as compared to a nonwoven material produced under same conditions from fibers produced under same conditions except for absence of the polymeric bond curve enhancing agent, with the raising of at least some points of cross-directional strength preferably including raising of peak cross-directional strength or raising at least some points at temperatures lower than peak cross-directional strength.
In still another aspect of the present invention, the polymeric bond curve enhancing agent provides an increase in area over a defined temperature range under a bond curve of cross-directional strength vs. temperature as compared to a nonwoven material produced under same conditions from fibers produced under same conditions except for absence of the polymeric bond curve enhancing agent. The increase in area can be provided by the bond curve being flatter and having the same, substantially the same or a lower peak cross-directional strength as compared to a nonwoven material produced under same conditions from fibers produced under same conditions except for absence of the polymeric bond curve enhancing agent. The increase in area can also be provided by the bond curve being of the same or substantially the same shape and having higher cross-directional strengths over at least some points on the bond curve over the defined temperature range as compared to a nonwoven material produced under same conditions from fibers produced under same conditions except for absence of the polymeric bond curve enhancing agent, with the at least some points preferably including a higher peak cross-directional strength. The increase in area can also be provided by the bond curve being shifted to lower temperatures with the area under the bond curve in the defined temperature range being increased as compared to a nonwoven material produced under same conditions from fibers produced under same conditions except for absence of the polymeric bond curve enhancing agent. The increase in area can also be provided by the bond curve being flatter and having cross-directional strength points at temperatures lower than peak cross-directional strength raised as compared to a nonwoven material produced under same conditions from fibers produced under same conditions except for absence of the polymeric bond curve enhancing agent. The increase in area can also be provided by the bond curve being flatter and being shifted to lower temperatures as compared to a nonwoven material produced under same conditions from fibers produced under same conditions except for absence of the polymeric bond curve enhancing agent. The increase in area can also be provided by the bond curve being flatter, being shifted to lower temperatures and having cross-directional strength points at temperatures lower than peak cross-directional strength raised as compared to a nonwoven material produced under same conditions from fibers produced under same conditions except for absence of the polymeric bond curve enhancing agent.
In the embodiments of the present invention, the polymeric bond curve enhancing agent preferably has (a) a DSC melting point of below about 230xc2x0 C., preferably below about 200xc2x0 C., more preferably a DSC melting point below that of the polypropylene in the polymer blend, and more preferably a DSC melting point of about 15 to 100xc2x0 C. below that of the polypropylene in the polymer blend, and (b) at least one of an elastic modulus and a complex viscosity below that of the polypropylene in the polymer blend. Preferably, both of the elastic modulus and the complex viscosity are below that of the polypropylene in the polymer blend, with the elastic modulus of the polymeric bond curve enhancing agent preferably being about 5 to 100%. below that of the polypropylene in the polymer blend, and the complex viscosity of the polymeric bond curve enhancing agent preferably being about 10 to 80% below that of the polypropylene in the polymer blend.
The polypropylene can comprise at least about 80 percent by weight of the polymer blend, more preferably at least about 90 percent of the polymer blend, with the polymer blend preferably comprising up to about 20 percent by weight polymeric bond curve enhancing agent, preferably up to about 10 percent by weight of the polymeric bond curve enhancing agent, more preferably less than 10 percent by weight, more preferably about 0.5 to 7 percent by weight, even more preferably about 1 to 5 percent by weight, even more preferably about 1.5 to 4 percent by weight, and a preferred amount being about 3 percent by weight.
The polymeric bond curve enhancing agent preferably comprises at least one polymer selected from the group consisting of alkene vinyl carboxylate polymers, polyethylenes, alkene acrylic acids or esters, alkene co-acrylates, acid modified alkene acrylates, alkene acrylate acrylic acid polymers, and polyamides. More preferably, the polymeric bond curve enhancing agent comprises at least one polymer selected from the group consisting of ethylene vinyl acetate polymers, polyethylenes, ethylene methacrylic acids, ethylene N-butyl acrylate glycidyl methacrylate, alkene co-acrylate co-carbon monoxide polymers, acid modified ethylene acrylates, ethylene acrylate methacrylic acid terpolymers, and nylon 6. Even more preferably, the ethylene vinyl acetate polymers comprise at least one of ethylene vinyl acetate copolymer and ethylene vinyl acetate terpolymer; the alkene co-acrylate co-carbon monoxide polymers comprise ethylene N-butyl acrylate carbon oxides; and the acid modified ethylene acrylates comprise at least one of ethylene isobutyl acrylate-methyl acrylic acid and ethylene N-butyl acrylic methylacrylic acid.
In the case of the polymeric bond curve enhancing agent comprising ethylene vinyl acetate polymer, it is preferably present in about less than 10 percent by weight. Additionally, the ethylene vinyl acetate polymer can contain about 0.5 to 50 weight percent vinyl acetate units, more preferably about 5 to 50 weight percent vinyl acetate units, even more preferably about 5 to 40 weight percent vinyl acetate units, even more preferably about 5 to 30 weight percent vinyl acetate units, with preferred more specific amounts being about 9 weight percent vinyl acetate units and about 28 weight percent vinyl acetate units.
The polymer blend can include additional polymers to polymers that are polymeric bond curve enhancing agents, such as polyethylenes, polyamides and polyesters. The polyethylene can have a density of at least about 0.85 g/cc, with one preferred range being about 0.85 to 0.96 g/cc, and an even more preferred range being about 0.86 to 0.92 g/cc.
Mixtures of polymeric bond curve enhancing agents can be preblended and/or additional polymers can be preblended with at least one polymeric bond curve enhancing agent to form a preblend, and the preblend can be mixed with the polypropylene. However, any other order of mixing can be used. The additional polymer can comprise various polymers, such as polyethylene, in amounts up to about 20 weight percent of the polymer blend.
The polymer blend can be prepared by various techniques, such as by tumble mixing.
The skin-core structure can comprise a skin showing an enrichment of ruthenium staining of at least about 0.2 xcexcm, more preferably at least about 0.5 xcexcm, more preferably at least about 0.7 xcexcm, even more preferably at least about 1 xcexcm, and even more preferably at least about 1.5 xcexcm.
With fibers having a denier less than 2, another manner of stating the ruthenium enrichment is with respect to the equivalent diameter of the fiber, wherein the equivalent diameter is equal to the diameter of a circle with equivalent cross-section area of the fiber averaged over five samples. More particularly, for fibers having a denier less than 2, the skin thickness can also be stated in terms of enrichment in staining of the equivalent diameter of the fiber. In such an instance, the enrichment in ruthenium staining can comprise at least about 1% and up to about 25% of the equivalent diameter of the fiber, preferably about 2% to 10% of the equivalent diameter of the fiber. Still further, the skin-core structure of the instant invention can be determined using a hot stage test, and a skin-core structure is present when a residue trail is present.
The polymer blend can include various additives, such as stabilizers, antioxidants, pigments, antacids and process aids. Various finishes can be applied to the fibers to maintain or render them hydrophilic or hydrophobic. Also, a component can be included in the polymer blend for modifying the surface properties of the fiber, such as to provide the fiber with repeat wettability.
The process can include feeding the polymer blend comprising the polypropylene and the polymeric bond curve enhancing agent, preferably ethylene vinyl acetate polymer to at least one spinnerette; and the extruding can comprise extruding the polymer blend through the at least one spinnerette.
The present invention is also directed to a process for preparing a fiber having a skin-core structure, comprising extruding a polymer blend comprising polypropylene and a softening polymeric additive as a hot extrudate; and providing conditions so that the hot extrudate forms a fiber having a skin-core structure.
The present invention is also directed to fibers, such as any fibers that are produced using any of the processes of the invention as well as fibers that have the structure and/or compositions that are described herein.
Thus, in one aspect, the present invention is directed to a fiber comprising a polymer blend of polypropylene and polymeric bond curve enhancing agent, preferably ethylene vinyl acetate polymers, with the fiber comprising a skin-core structure, and the polypropylene and the polymeric bond curve enhancing agent being present in both the skin and the core of the skin-core structure.
The fiber can have can various cross-sectional configurations, such as circular, diamond, delta, concave delta, trilobal, oval, or xe2x80x9cXxe2x80x9d-shaped, and is preferably of circular or concave delta cross-section configuration.
The fiber in accordance with the present invention can be continuous and/or staple fiber of a monocomponent or bicomponent type, and preferably falls within a denier per filament (dpf) range of about 0.5-30, or higher, more preferably is no greater than about 5, and preferably is about 0.5 and 3, more preferably about 1 to 2.5, with preferred dpf being about 1.5, 1.6, 1.7 and 1.9. The fiber can include at least one hollow portion.
In the fiber, the polypropylene can comprise a dominant phase of the skin-core structure, and the polymeric bond curve enhancing agent can comprise fibrils dispersed throughout the skin-core structure, and therefore present in both the skin and the core.
In one aspect of the fiber, the skin-core structure can comprise a surface zone, an inner zone and a gradient therebetween, with the surface zone comprising a high concentration of oxidative chain scission degraded polypropylene as compared to the inner zone, and the gradient comprising a decreasing weight average molecular weight towards the external surface.
In another aspect of the fiber, the skin-core structure can comprise an inner core of the polymer blend, and a surface zone of the polymer blend surrounding the inner core, with the surface zone comprising the polymer blend as oxidative chain scission degraded polypropylene, so that the inner core and the surface zone define a skin-core structure of the polymer blend. Further, the oxidative chain scission degraded polypropylene can be substantially limited to the surface zone, wherein the inner core and the surface zone comprise adjacent discrete portions of the skin-core structure.
The fiber can comprise a skin-core structure including an inner core and a surface zone having a thickness of at least about 0.2 xcexcm, more. specifically at least about 0.5 xcexcm, more specifically at least about 0.7 xcexcm, even more specifically at least about 1 xcexcm, and even more specifically at least about 1.5 xcexcm surrounding the inner core, with the inner core comprising the polymer blend and the surface zone comprising the polymer blend as oxidative chain scission degraded polymeric material. Further, the oxidative chain scission degraded polymeric material can be substantially limited to the surface zone, wherein the inner core and the surface zone can comprise adjacent discrete portions of the skin-core structure. Alternatively, there can be a gradient of oxidative chain scission degraded polymeric material between the inner core and the surface zone.
Still further, as discussed above, for fibers having a denier less than 2, another manner of stating the ruthenium enrichment is with respect to the equivalent diameter of the fiber. More particularly, for fibers having a denier less than 2, the skin thickness can also be stated in terms of enrichment in staining of the equivalent diameter of the fiber. In such an instance, the enrichment in ruthenium staining can comprise at least about 1% and up to about 25% of the equivalent diameter of the fiber, preferably about 2% to 10% of the equivalent diameter of the fiber.
In another aspect of the fiber according to the present invention, the skin-core structure can comprise an inner core of the polymer blend, and a surface zone surrounding the inner core, with the surface zone comprising the polymer blend as oxidative chain scission degraded polymeric material, so that the inner core and the surface zone define a skin-core structure, and the inner core can have a melt flow rate substantially equal to an average melt flow rate of the fiber.
In still another aspect of the fiber according to the present invention, the skin-core structure can comprise an inner core of polymer blend having a melt flow rate, and the fiber can have an average melt flow rate about 20 to 300% higher than the melt flow rate of the inner core.
The fiber according to the present invention is also preferably characterized by various parameters utilizing terminology that will be defined in the detailed description, and is briefly indicated in this section.
Thus, in another aspect of the present invention, the fiber preferably has a %xcex94A1 which is greater than that of a nonwoven material produced under same conditions from fibers produced under same conditions except for absence of the polymeric bond curve enhancing agent. Preferably, the %xcex94A1 is increased by a member selected from the group consisting of at least about 3%, at least about 15%, at least about 20%, at least about 30%, at least about 40%, at least about 50% and at least about 60%.
Still more preferably, the fiber has a %xcex94A1 and a %xcex94Am which is greater than that of a nonwoven material produced under same conditions from fibers produced under same conditions except for absence of the polymeric bond curve enhancing agent. Even still more preferably, the fiber has a %xcex94A1, a %xcex94Am and a %xcex94Ap which is greater than that of a nonwoven material produced under same conditions from fibers produced under same conditions except for absence of the polymeric bond curve enhancing agent.
The polymeric bond curve enhancing agent can comprise a plurality of polymeric bond curve enhancing agents. For example, the plurality of bond curve enhancing agents can comprising at least one ethylene vinyl acetate polymer and at least one polyamide, or at least one ethylene vinyl acetate polymer and at least one polyethylene.
The present invention is also directed to skin-core fiber containing polypropylene and polymeric bond curve enhancing agent which when processed into a nonwoven material by thermal bonding obtains for the nonwoven material at least one of a Cm of at least about 60%, more preferably at least about 75%, and even more preferably at least about 90%; a Cp of at least about 75%, and preferably at least about 90%; a C1 of at least about 50%, more preferably at least about 70%, and even more preferably at least about 90%; a R1 of at least about 55%, preferably at least about 70%, more preferably at least about 80%, still more preferably at least about 85%, still more preferably at least about 90%, and even more preferably at least about 95%; and a Rm of at least about 90%.
The present invention is also directed to a skin-core fiber containing polypropylene and polymeric bond curve enhancing agent which when processed as a fiber into a nonwoven material by thermal bonding obtains for the nonwoven material at least one of an Am of at least about 3000, preferably at least about 5000, even more preferably at least about 6000 and even more preferably at least about 7000; an Ap of at least about 2500, preferably at least about 3500, even more preferably at least about 6000, and even more preferably at least about 6500; and an A1 of at least about 2500, preferably about 6000, even more preferably at least about 7500, even more preferably at least about 9000, and even more preferably at least about 10000.
The invention is also directed to a skin-core fiber comprising polypropylene and a polymeric bond curve enhancing agent, preferably ethylene vinyl acetate polymers, the polypropylene and the polymeric bond curve enhancing agent being formed into the skin-core fiber under fiber processing conditions, and the skin-core fiber when processed into a thermally bonded nonwoven material under nonwoven processing conditions obtains, with respect to a nonwoven material produced under the same nonwoven processing conditions from fiber produced under the same fiber processing conditions but not containing the polymeric bond curve enhancing agent, at least one of a xcex94Cm of at least about 3%, preferably at least about 10%, more preferably at least about 20%, still more preferably at least about 30%, still more preferably at least about 40%, still more preferably at least about 50%, and even more preferably at least about 60%; a xcex94C1 of at least about 3%, preferably at least about 10%, more preferably at least about 20%, still more preferably at least about 30%, still more preferably at least about 40%, still more preferably at least about 50%, and even more preferably at least about 60%; a %xcex94m of at least about 3%, preferably at least about 10%, more preferably at least about 20%, still more preferably at least about 30%, and even more preferably at least about 40%; a %xcex94A1 as discussed above; a xcex94Rm of at least about 3%, preferably at least about 10%, more preferably at least about 20%, still more preferably at least about 25%, and even more preferably at least about 30%; and a xcex94R1 of at least about 3%, preferably at least about 10%, more preferably at least about 20%, still more preferably at least about 30%, still more preferably at least about 35%, and even more preferably at least about 40%.
The present invention is also directed to nonwoven materials comprising fibers as described herein which are bonded together, preferably thermally bonded together; and to hygienic products including these nonwoven materials and at least one absorbent layer. One such hygienic article is a diaper comprising an outer layer, an inner nonwoven material, and an intermediate absorbent layer. The nonwoven material of the invention can be used as the outer layer, which can be an outer impermeable layer but can also be permeable, and/or the inner nonwoven material. Also, the present invention is directed to fibers produced by the processes described herein.
The nonwoven material preferably has a basis weight of less than about 20 g/yd2 (gsy), more preferably less than about 18 g/yd2, more preferably less than about 17 g/yd2, even more preferably less than about 15 g/yd2, more preferably less than about 14 g/yd2, and even as low as 10 g/yd2, with a preferred range being about 14 to 20 g/yd2.
The fibers of the present invention provide superior bond strength compared with conventional polypropylene fiber. The nonwoven materials of the invention exhibit superior cross-directional tensile properties, elongation and toughness. Further, nonwoven materials produced with the fibers of the present invention have uniformity, loftiness, opacity and softness. Most-notably, the fibers produce nonwoven material having (a) a flattened bonding curve, (b) raising of the bonding curve, i.e., increase in cross-directional strength and/or (c) shifting to the left of the bonding curve, i.e., to lower temperatures, of cross-directional strength vs. bonding temperature of a nonwoven material, so that the strength properties of the nonwoven material, especially the cross-directional strength, are maintained or increased with a skin-core fiber.